How long does it take for logging roads to recover from clearance?

Roads are generally terrible for biodiversity. They fragment habitat, can increase habitat loss and hunting as a result of increased access, and cause direct loss of biodiversity as a result of collisions. However, not all roads are the same. Some are massive, permanent structures, while others are temporary, dirt tracks that may seemingly disappear once they fall into disuse.

One example of ephemeral roads is those that logging companies construct in tropical forests to provide access and transport of logs. There has long been concern that these roads can increase the risk of fires occurring, as well as increasing access for hunting, and other forms of forest exploitation. However, in a recent(ish) paper* has shown that some of the negative effects of logging roads are relatively transient.

In the paper, Fritz Kleinschroth and colleagues showed that in Central African forests, after 30 years of recovery logging roads had similar canopy cover, species diversity, and leaf litter to logged forests nearby* . However, the amount of carbon stored in the form of biomass lagged behind and was only 6% of that seen for logged forests after 30 years of recovery. At this rate, biomass recovery would take more than 300 years.This incredibly slow recovery at first appears puzzling, given that secondary forests, which have had almost all their trees cut down in the past and turned into agricultural fields, tend to take between 60-100 years to recover biomass to pre-disturbance levels (see here for a blog post and here for a recent paper on this). However, the probable cause of this delayed recovery is the compaction of soils on the roads by heavy vehicles which reduces seed germination and root growth***. Taken together the authors suggest these results indicate that logging roads have the potential to act as areas in which timber species could regenerate and that they may become inaccessible to hunters within 10 years.

So how does this study compare to similar ones carried out previously? Firstly, this study is a little different because it is one of the few that used chronosequences to assess recovery, and so the only study I know of which can assess longer-term dynamics on logging roads. However, other studies present a similar picture for the recovery of biomass and forest structure – forest canopy cover recovers relatively quickly (see here and here), but biomass and basal area lag behind (see here and here). The major difference between this study and previous ones is that it presents a more optimistic outlook of biodiversity. Previous studies have estimated that species richness may be 50-95% lower on abandoned logging roads when compared to logged forests (see here, here, and here). As such, the relatively fast recovery of species richness shown by Kleinschroth and colleagues appears to be outside of the norm, and further similar studies will be needed to see whether the pattern of recovery shown in this paper is an outlier. So we can’t really give a solid answer to the question posed in the title of this post – sorry about that.

While results vary from study to study it is obvious that more efforts need to be made to reduce the number of logging roads, their initial impacts to forests, and to help them recover once they are abandoned. In order to reduce the number of roads, reduced-impact logging could be used. This method, which I have been accused of disliking in the past, seems to be very successful in reducing the number of roads in logging concessions where it has been used (see here for an example of this). This is done by producing a plan for road construction prior to any trees being cut, rather than the ad-hoc approach often taken in conventional logging. Reducing the impact of roads could be done by limiting their width. Finally, improving recovery could be done by planting seedlings/saplings on former logging roads, as well as reducing access to roads.

One suggestion that the authors made in their paper that I really like is to re-use logging roads when forests are re-logged. Given that logging typically occurs every 30 years, this would allow some time for recovery of biodiversity on the roads but clearing them would reduce the damage caused by their construction spreading to other areas of the forest.

*I admit it, I’ve been terrible at keeping up with my reading recently.

**John Healy and Fritz have written a nice summary of their paper on the website “The Conversation” which is well worth a read.

***Anecdotal, I know, but I have seen similar things on restored salt marshes in the UK where diggers have been used to breach sea walls. At least for the ones I remember, this resulted in reduced vegetation cover.


Second growth:The promise of tropical forest regeneration in an age of deforestation


Anyone who knows anything about secondary forests will have come across the work of Robin Chazdon. She has inspired at least one forest ecologist, me, that forests recovering from major disturbances are a subject worthy of study. I’m sure she has done the same for many others out there. So, coming towards the end of my PhD I was excited to see a book that she had written summarising the topic was due to be released and using the last of my NERC funds I bought it. And then I moved house to Spain, where the book sat untouched and unloved in a box for the next year. After I came back to the UK last year, I found the book again and decided I should stop putting off reading it. I read it on trains, buses, on my sofa and occasionally in bed. I once fell asleep reading this book, though admittedly that was on the way back from the BES annual meeting  in Edinburgh, and the gin from the previous night was probably the cause of my sleepiness rather than any bad writing.

The first thing to say is that this book is extremely comprehensive. Though it is not particularly lengthy, running to 316 pages of text, it covers a huge range of topics relating to forest regeneration from traditional knowledge and prehistoric forest transformations by humans to recovery pathways from fire, landslides, volcanic eruptions, logging, and agricultural use. There are also numerous sections on community assembly, functional traits, ecosystem function, and animal and plant interactions. The last section concentrates on reforestation and restoration of degraded forests, making a passionate plea for degraded forests to not be considered as wasteland.

For me the most fascinating parts of the book were those that covered traditional knowledge of forest regeneration and the history of human cultures in tropical forests – both subjects I knew practically nothing about before this book. I was captivated to read that the dayak people of Borneo have five words to define different stages of forest recovery – kurat uraq (1-3 year old scrub that forms after abandonment), kurat tuha (trees > 5 cm in diameter and 5-6 metres in height), kurat batang muda (trees 10-15 cm in diameter), kurat batang tuha (closed canopy secondary forest) and hutan bengkar (primary forest). As Chazdon points out this knowledge shows a striking resemblance to that of forest ecologists. Similarly, Mayan cultures in Central America and Soliga people in the Western Ghats have developed a subtle knowledge of the stages of forest succession. I have always been a bit skeptical of integrating traditional knowledge into ecological science, but this book convinced me that there could be some value to it.

Chazdon masterfully weaves together anthropology, archaeology and ecology in the discussion of prehistorical impacts of humans on tropical forests. She cites evidence of earthworks called geoglyphs similar to the Nazca lines found in the state of Acre in Brazil, swidden agriculture 20,000 years ago in Papua New Guinea and human populations in Central America to dispel the view that any forest is truly untouched. There are probably legacies of human use in most forests, we just can’t identify them. Based on this she, perhaps controversially, critiques recent work suggesting that mature tropical forest biomass density is increasing as a result of atmospheric carbon dioxide. Chazdon’s view is that this increase could well be as a result of recovery from unseen disturbances that happened generations ago.

The section on community turnover during succession is also excellent, with a detailed analysis of the characteristics of short- and long-lived pioneer and shade tolerant, late successional species. At points Chazdon playfully conjures up text resembling Shakespeare’s  “All the world’s a stage” monologue: “The term successional stage is apt. Successional pathways can be viewed as an improvisational drama in several acts, with each act featuring a different set of actors. Some actors perform throughout the drama, but others have cameo appearances of only one act. Although each act sets the stage for the next, forest regeneration has no director and only a roughly sketched script creating a high degree of spontaneity, randomness and uncertainty.” These are amongst my favourite parts of the book, with metaphor mixing with a solid science to help things stick in your mind that might otherwise be easily forgotten.

If I have any criticism of the book, it is that it’s a bit repetitive. This is probably because Chazdon sees succession as ‘an improvisational drama in several acts’ and so the book relies on case studies, rather than synthesising current knowledge to form generalities. However, I think that the repetition helps if you just want to dip in and out of chapters – I don’t think it is necessarily written to be read cover to cover like I did.

That aside if you are interested in the dynamics of forests in any way this book is essential reading. There is no better summary of current thinking on tropical forest succession out there.

Does reduced impact logging in tropical forests benefit carbon storage and species richness?

After a bit of a traumatic review process* we have just had a paper published in Forest Ecology and Management on the impacts of tropical selective logging on carbon storage and tree species richness. I’m really pleased that we finally got this work out there. If you want to give it a look you can get it here.

Selective logging is one of the most widespread drivers of tropical forest degradation. As I have said before around 400 million hectares of tropical forest are now used for logging – an area twice the size of Russia. Or one hundred and ninety two and a half times the size of Wales – if that’s your thing**.

High intensity logging can result in loss of animal species richness, but on the whole logging is seen as one of the least damaging human uses of tropical forests. That said, there are still concerns about its sustainability in the long-term. Poorly managed concessions commonly remove high timber volumes and do not leave enough time between logging cycles to allow forests to recover.

To improve the sustainability of the practice, reduced impact logging has been proposed. This method aims to reduce negative environmental impacts by cutting lianas and vines before logging, identifying which trees to cut and mapping them before logging starts, planning the roads to be built, and training staff in methods to reduce damage to the forest.  The first papers testing this method showed promising results, appearing to indicate that reduced impact logging causes lower carbon emissions when compared to conventional methods.

However, many papers that have  looked at the impacts of reduced impact logging failed to account for the volume of wood taken out of forests. Crucially, if this differs between reduced impact and conventionally logged sites this represents a hidden treatment, which if not accounted for can lead to faulty conclusions. Given that there are calls to pay people who use reduced impact logging as a means to reduce carbon emissions, we need good, solid science to support this policy.

So, we tried to solve the question of whether reduced impact logging still reduces negative effects on residual tree damage, aboveground biomass, and tree species richness using meta-analysis. We compiled data from all over the globe, all from previously published papers.

Locations of study sites where data we used was collected
Locations of study sites where data we used was collected

Cutting to the chase, the results for reduced impact logging were mixed.

It seemed to reduce the damage to residual trees once logging volume was accounted for…

Reduced impact logging (blue) tended to cause less residual damage than conventional logging (red) once logging intensity was accounted for

… however, this did not obviously result in reduced biomass losses, and evidence of an effect on tree species richness was poor as well.

Effects of logging intensity on (a) aboveground biomass and (b) tree species richness. Reduced impact logging sites are blue points, and conventional sites are red. Note the relatively low intensity for most reduced impact logging sites.

Though residual damage to trees was reduced, this didn’t cause a  reduction in overall biomass loss. This may be the result of a few different factors. Firstly, residual damage is often to smaller trees so it is not necessarily that surprising that this had little effect on biomass. Secondly, we are really lacking enough data to be sure of the relationship between biomass changes and reduced impact logging. Nearly all of the data is from forests logged at low intensity so we cannot say if the slope of the relationship differs from that of conventional logging.

In the case of tree  species richness, the relative lack of change over a gradient of logging intensity is not too surprising. Newly logged areas richness is probably enhanced by fast growing pioneer species. However, richness is not a fantastically useful measure of biodiversity, in the future it would be much more useful to be able to say what type of species are being lost/gained not just the total number of species in a site (see the recent paper by Zuzana Burivalova and colleagues that tries to do this with bird species).

So what does this all mean? Does our study mean that reduced impact logging doesn’t work? The short answer is no. The long answer is a bit more complicated than that.

First we need to decide whether reduced impact logging is synonymous with low yield logging. If it is then that is fine, but we need to be upfront about this. Logging is after all mainly about timber production. However,some people have previously argued that reduced impact logging can reduce damage whilst maintaining yields. If this is true, it would represent a win-win situation.

If we decide that reduced impact logging isn’t synonymous with low yields then our research question needs to change from “Does reduced impact logging cause less damage than conventional logging?” to “How do the impacts of reduced impact and conventional logging vary over a gradient of timber yields?” Generally in ecology we focus too much on using categorical x variables in statistical tests, and this case is a great example of why this approach can hold back our science (see the fantastic post by Brian McGill on this subject here).

Previous studies show that animal species richness declines with increasing logging intensity and reduced impact logging causes lower losses of animal populations. As a result, a combination of reduced impact logging and reduced logging intensity may appear the best way to reduce carbon emissions and biodiversity loss from logging. However, reducing local yields may cause expansion of logging into previously unlogged areas. This mirrors the current land sharing/sparing debate on how to balance agricultural yields and food production. This debate is taking off regarding logging, and I am keen to see more work on tropical logging that acknowledges the importance of yields. As I said to someone at a conference recently, if we ignore the importance of logging yields why study logged forests?

However, to inform this debate we need more powerful tests of different logging methods than we could do in our paper. One possible source of data for this are studies where logging intensity has been calculated for each sample plot used. For most of the studies I used logging intensity was only available at the site level. Getting this detail would give more statistical power to our tests and provide a more solid evidence base for management of tropical forests. Large-collaborative projects such as the tropical managed forests observatory represent a great chance to answer this question in a more satisfactory manner.

*I will write more about this next week.None of the journals were to blame, just some very biased reviewers.

**If any US citizens want this calculating as relative to Rhode island, I did it. It’s 1273.8 Rhode islands.

Impacts of selective logging of tropical forests on tree damage, biomass and tree species richness


A few days ago we put some of my thesis work on the impacts of tropical forest logging on the preprint server PeerJ. The work is currently in review elsewhere but I thought I should put up a blog post about our findings, even if they change a bit after the review. I am really pro the idea of making results available as soon as possible so that they can be read and cited. I have lost count of the number of times a piece of work I have seen presented at a conference and wanted to cite has taken 1-2 years to come out as a paper. In short I think preprints are the future, so feel free to read, comment on and critique ours over at Peerj (and you can even cite it if you like).

A fifth of tropical forests have been logged in the recent past. Though logging is an important source of timber and jobs it also faces questions about its long-term sustainability particularly in its impacts on biodiversity and carbon. However, as I have written before, the results of studies on the impact of logging are very variable making generalisation difficult. Previous meta-analyses of the impacts of logging have indicated that biomass losses can be as high as 66% or as low as 4%, while tree species richness may be reduced by as much as 53% or show increases of up to 27%. However, none of these meta-analyses of the impacts of logging on tree biodiversity or biomass have explored the potential reasons for these differences. A recent study showed that differences in the impact of logging on animal species richness was explained by the variation in intensity of logging at sites, measured as the volume of wood removed per hectare. Interestingly this study showed that while species richness generally declined for amphibians, mammals and invertebrates with increasing intensity, bird species richness actually increased slightly.

Another recent piece of work by Jake Bicknell and colleagues has shown that a method called reduced impact logging (RIL), a technique which aims to reduce logging damage by altering extraction methods, reduces the negative effects of conventional logging techniques on animal population sizes. This is an important result as RIL has long been championed as a potential solution to the problems of logging sustainability and Jake’s work is the first to really show that it has positive effects across a number of sites.

So while two recent meta-analyses have indicated that logging intensity and method may have a profound influence on forest biodiversity no similar work has been done for trees, despite the fact that programmes that focus on conservation of carbon, such as REDD+, need this evidence to implement policy. So to fill this knowledge gap we performed a meta-analysis to determine what factors relating to logging intensity and method may cause variation in the impacts of logging on residual tree damage, aboveground biomass and tree species richness. In total we collected data from 62 studies across the tropics giving us 38, 43 and 9 data points for investigation of damage, biomass change and species richness change respectively. Prop_damaged_vol Promisingly we found that RIL seemed to reduce residual tree damage compared to conventional logging, with greatest differences to conventional logging found at low intensities. However, at higher intensities residual damage became more similar to that of conventional logging as suggested by previous work from Indonesia. Prop_volume2However, the same wasn’t true of biomass. Though there was an apparent statistical difference in the slopes for RIL and conventionally logged forests the relatively low overlap in the intensities at which conventional and RIL are carried out means that slope estimation is not fantastic. As such it is not entirely clear whether, at the stand scale, RIL reduces biomass losses because of lower intensities or differences in practice as the two are confounded. SR_volumeUnfortunately there wasn’t enough information on species richness from forests logged using RIL to allow a comparison. However, the results were still interesting. There was a general decline in species richness with increasing logging intensity, but the plot hints that richness might increase at low intensities. Whether or not this is as a result of intermediate disturbance hypothesis type relationships is not a fight I want to get into, but this work does confirm that tree species richness is relatively insensitive to logging even at high intensities.

So our work suggests that the evidence for the positive effects of RIL is mixed, once we account for differences in logging intensity.  I am well aware that this piece of work might annoy a few people who think I have something against RIL. Those I have spoken to at conferences where I have presented often think that I am saying that RIL doesn’t work. I’m not. It just isn’t entirely clear what it’s effects are. Frankly it would be remarkable if RIL and conventional logging had similar impacts at the stand scale given the differences in the two practices. What I think we lack is enough evidence to say what is going on.

So what would be a constructive way to determine the differences in impact of RIL and conventional logging? One thing we think would improve evidence is the quantification of logging intensity at the plot scale. Currently studies often report logging intensities for the entire landscape where plots are located, meaning that the variation between plots is not accounted for. There is likely to be a big difference amongst plots and so the impacts are likely to differ as well. As far as I can tell only a few studies have done this. One good example is the work of Lucas Mazzei and colleagues who showed that plots that had been more intensively logged showed a slower recovery in biomass. Metrics such as the basal area of trees removed per hectare might be useful and relatively easy to collect at the plot scale.

The results of our study and those of Zuzana Burivalova suggest that logging intensity drives carbon and species loss while Jake Becknell’s work suggest that RIL is less damaging for animal populations. As such, current evidence suggests that RIL at relatively low intensities is likely to be the best way to reduce carbon and biodiversity loss in tropical logged forests. However, given the massive area of tropical forest already designated for logging reductions in local intensity, and thus yield, may encourage expansion into previously unlogged areas. Recent work indicates that high intensity logging over a smaller area (‘land sparing’) may have better outcomes for tropical forest species than low-intensity extensive timber extraction (‘land sharing’) in Borneo, though there is a need for similar studies in other areas of the tropics. Although reductions in logging intensity may reduce impact, the high demand for timber requires novel solutions that do not drastically reduce current yields but reduce impacts on forest ecosystems. Methods such as silvicultural thinning techniques to remove pioneer species may aid recovery of floral community composition, carbon and timber stocks but further work is needed to assess their effectiveness. Although RIL may also provide a solution, further evidence is required to verify this for carbon storage in the form of above-ground biomass. Analyses that take into account plot level variation in logging intensities using collaborative networks such as The Tropical managed Forests Observatory offer a potential solution to this.

If you enjoyed this be sure to check out the preprint on peerJ or the posts ‘Logging intensity drives species richness loss’ and ‘How bad is logging for tropical biodiversity?’

Logging intensity drives species richness loss

An area bigger than the entire Indian landmass is now used for timber production in the tropics. This logging is largely selective and leads to degradation with loss of specialist species and ecosystem services like carbon storage. However, many have also argued that these forests should be considered a priority for protection since they are at danger of conversion to other land-uses such as agriculture. In addition the impact of logging on tropical forest biodiversity appears to be less negative than other human impacts.

However, simply saying that logging is a less damaging option when compared to other way in which humans exploit forests misses a lot of what is going on. Logging operations differ massively from place to place in terms of the volume of trees cut for timber, the area affected by logging, the distance between logged and unlogged areas, I could go on… All of these differences have the potential to influence the the effect of logging on biodiversity.

Continue reading

Forest regeneration provides cheap carbon and biodiversity benefits

First of all, hello again and apologies for my sporadic posting on here recently. I have now successfully defended my viva and have a few corrections to make but hopefully should be able to post on here a bit more regularly from now on.

One paper I read that really impressed me while on my hiatus from the blog was by my old commuting buddy James Gilroy and colleagues. This paper attempted to identify the potential biodiversity and carbon benefits of forest recovering in the Tropical Andes in Colombia, an area full of species found nowhere else many of which are under threat from agricultural conversion. The paper also attempted to look at the cost effectiveness of carbon payments for landowners who converted farmland to forest when compared to different land-use options like cattle farming.

Gilroy et al - Fig 1
Recovery of secondary forest carbon stock compared to that of pasture and primary forest (Taken from Gilroy et al. 2014)

I was actually quite surprised by what Gilroy and his team found. Their results suggested that carbon storage in recovering forests was fairly similar to that in mature forests in the area after around 30 years, much less than the 100 years or so that I estimated these stocks would take to recover in a previous study.

Gilroy et al - Fig 4

Gilroy et al - Fig 3
Relationships between carbon stocks and similarity of dung beetle and bird communities to primary forest communities (Taken from Gilroy et al. 2014)


More surprising still was that bird and dung beetle communities in the regenerating forests were fairly similar to those of mature forests, suggesting that they have high conservation value. Again previous studies have generally estimated that animal species that are forest specialists may take a long time to colonise secondary forests, and plants probably take even longer. The fast recovery times may be attributable to the relative closeness of recovering forest to intact forests in the study area, allowing immigration of  forest animals and increased likelihood of transportation of seeds from long lived tree species.

Gilroy et al - Fig 2
Relationship between the additional cost of undertaking forest regeneration and the price paid for carbon per tonne. The solid horizontal line shows where costs are equal to zero. This graph indicates that there are potentially net economic benefits for people undertaking forest regeneration projects when the carbon price is greater than $4 per tonne. (Taken from Gilroy et al. 2014)

More important than these findings though was the discovery that if forest regeneration schemes were implemented in the area, they could be more profitable to land-owners than current land-uses like cattle farming. This was true for all pastures in the area when carbon trading prices were greater than $4 per tonne of CO2 and given that the median price of carbon in 2013 was around $7.80 per tonne, paying for the carbon benefits of regeneration in these locations works out cheaply. This is the part that I thought was really neat, because all too often restoration schemes fail to account for the costs and benefits associated with such projects.

Given that the study area has fairly representative socioeconomic conditions to those found in the wider Colombian Andes, the results suggest that regeneration of cloud forest may provide a great opportunity for REDD+ carbon based conservation, which can deliver multiple environmental benefits at minimal cost. Though REDD+ has its critics it has the potential to transform forest conservation so we need to work hard to make sure it is done in the right way.

Why “zero deforestation” is bad for forests

Forest loss around roads in the Amazon (taken from

Over the last decade there has been increasing interest in reducing deforestation, spurred on by the Reducing Emissions from Deforestation and Degradation (REDD+) initiative.

It’s a great thing that reducing deforestation is now being discussed a bit more seriously by policy makers, but it can lead to some perverse situations – as a new article by Sandra Brown & Daniel Zarin in Science discusses. (I know this article is not so new anymore, it just took me 2 weeks to get round to writing this post, ok?)

Numerous governments and companies have set themselves goals of “zero deforestation.” In some cases this means net deforestation, in others gross deforestation and others haven’t really defined what they mean…

So why is this dangerous? I hear you ask.

Well for one thing, not all “forests” were created equal. Net deforestation measures include both the loss of forest from deforestation and the gains from forest regrowth and plantations. Plantation and recovering forests are not the same as undisturbed forests which contain more carbon and unique species. Under this definition all native forests could be lost from an area and replaced with plantation without any apparent deforestation. Stupid, right?

Brown & Zarin suggest a way to get proper estimates of deforestation would be to build on the Brazilian system of a satellite monitoring of deforestation that is clear and transparent. In Brazil deforestation statistics only account for forest loss and not any of the gains from regrowth or plantations. There is no reason why we can’t do this type of monitoring given the fantastic technology available that is now able to produce high resolution, global maps of forest change, like those below (go here to see more of these).

This slideshow requires JavaScript.

The paper also suggests practical ways to reduce the problems of deforestation in individual countries, depending on their development. Countries are commonly categorised as having little forest loss, accelerating forest loss, decelerating forest loss and reforestation – these are said to represent the stages of forest transition. Use of non-forest land for agriculture should be encouraged in countries where forest loss is high, as has been encouraged for palm oil in South East Asia previously. However, in countries with low forest loss but little non-forested land that can be cultivated setting zero deforestation targets is unreasonable, given expanding populations.

As a big idea zero deforestation sounds great. But next time you hear someone talking about it think about what they actually mean.  Gross deforestation and reforestation should be considered as separate aims, to avoid confusion. Without this “zero deforestation” is set to mean very little.